The present invention relates to the field of safety and accuracy in crude oil and natural gas well drilling, particularly the use of thermodynamic principles in order to develop a new method for safely and accurately conducting crude oil and natural gas well drilling.
Well blowouts are caused by the uncontrolled release of crude oil or natural gas well after a well's pressure control systems have failed. Given the threat to life and adverse impact on property and environment that a blowout can have, significant planning and precautions are undertaken when drilling the well. As part of safe drilling practices, persons who drill wells must consider several factors when planning a well, including pore pressure determination. The Pre-drill estimation of pore pressure and fracture gradient analysis is the bedrock of the well-planning process. Optimal pore pressure and fracture gradient estimates rely on the accuracy of overburden gradient calculations, which signify the characteristics of a given basin. If overburden or vertical stress gradient calculations are off, then pore pressure and fracture gradient estimates may be grossly underestimated or overestimated, both of which can result in severe wellbore instability and/or well control issues.
“Geopressure” refers to a subterranean earth formation where the fluid pressure of the pores exceeds hydrostatic. More specifically, Terzaghi's Principle states that all quantifiable changes in stress to a soil (e.g., compression, deformation, shear resistance) are a direct result of a change in effective stress. The effective stress σ′ is related to total stress σ and the pore pressure u by the relationship σ=σ′+u reading that total stress is equal to the sum of effective stress and pore water pressure.
Subsequently, the above authors defined the effective stress of a system as the difference between the total overburden of overlying sediments and the pressure of fluids occupying the pores of rock material. Later the oil and gas industry defined the “normal” pressure as the Salt water gradient or 0.465 psi/ft for the Gulf of Mexico and any pressure in excess of normal gradient as “abnormal” pore pressure.
Yet, an inherent weakness in Terzaghi's Principle is that it fails to incorporate additional thermally induced pressure into the equation. In addition, this theory does not relate the thermal properties of the rock formation and heat flow—that is specific to shale and sandstone formations—to the location and magnitude of Formation Pore Pressures. Terzaghi's approach to the compaction-dominated system does not need to take into account the effect of “high” Temperature and Thermal Conductivity of rock material but it would have been appropriate to have included the cold and low temperatures. This is because unusual pressures are often seen to affect the freeze-thaw cycles which in turn affect the shape of pores and in places such as Alaska affects certain surface pressures. For example, the contribution of temperature or geochemical reactions to the effective stress gradient is not represented in either Terzaghi's equation or the publications by the above mentioned authors.
Although thermodynamic or related properties associated with temperature are recognized by the petroleum industry; the experts, especially those engaged in serious basin modeling, have not used it in conjunction with the existing pore pressure estimation model. Because failure to properly calculate pore pressure can threaten the health and safety of workers and the environment, a method to properly mitigate these concerns and aid in the economic recovery of natural resources is desirable.